It has long been known that variations in the pressure of the material being molded in the cavity of a molding machine or cast in the cavity of a die cast machine is the most frequent cause of flash being formed in the parts being produced, short shots of material being fed to the cavity, and inconsistencies in the properties of the parts produced. With particular reference to the operation of molding machines, pressure variations occur because of changes in the viscosity of the material, i.e., plastic being molded. Such changes are commonly found to be due to either temperature variations or molecular weight. Pressure variations also occur as a result of variations in machine performance due to such things as timer errors, air temperature, hydraulic response, etc. In addition, pressure variations are attributable to operator errors wherein wrong timers, settings, etc., are employed by the operator.
Most rejects, it has been found, are caused by occurrences coming during the filling portion, i.e., when material is being fed to the cavity, and during the packing portion, i.e., when the material is solidifying in the cavity, of the molding cycle. It is this part of the cycle, i.e., the filling and packing portions thereof when orientation is developed, molecular packing is induced, and peak pressures are determined. Any errors or inconsistencies in the dynamics of filling and packing result in variations in the parts produced.
As the cavity fills with material, the pressure therein will rise slowly until packing of the material begins to occur, whereupon the pressure will rise rapidly. This may be referred to as the dynamic portion of the cycle of operation of the machine. If the pressure in the cavity rises too slowly or is stopped too soon, then short shots will occur because the material is afforded an opportunity to solidify before the cavity is entirely filled, or in the case of the mold having plural cavities, before all of the cavities are filled with material. If the pressure rises too rapidly or occurs too long, then flash is likely to form on the part being produced due to overpacking.
The likelihood that pressure variations will occur causing the undesirable results described above is particularly likely where thin-walled parts, i.e., lids, containers, housings, etc. are being produced inasmuch as the production of the latter requires that the cavity be filled quickly but not be overpacked. These two requirements however are often in conflict. As a result, short shots caused by insufficient pressure, flashed parts caused by excessive pressure, and warped parts caused by overpacking or underpacking are common.
In order to eliminate the existence of undesirable pressure variations in the mold cavity, a number of different types of control systems have been proposed for inclusion in molding machines as a means of effectuating control over the latter thereby to enable the amount of pressure in the cavity of the machine to be regulated. Because of the difficulty in controlling pressure, however, one approach which has been employed in the prior art for instance is the so-called volume-feeding technique wherein a limit switch or other equivalent control device is utilized to control the amount of material fed each cycle. Moreover, the screw by means of which the material is fed into the mold is caused to move through a predetermined distance. This method works as long as the limit switch is accurate and the nonreturn valve operatively associated with the screw does not leak. However, these two conditions rarely exist for long.
Another approach, similar to the aforereferenced volume-feeding technique, which has also been utilized heretofore is the technique of weight-feeding wherein the theoretical weight of the material needed to fill the mold completely is determined and this amount of material is caused to be fed to the mold once each cycle. Although the weight-feeding technique has been found to produce more accurate results than the volume-feeding technique, it also nevertheless suffers from a disadvantage. Namely, even a small deviation in the weight of the material fed to the mold can produce wide variations in cavity pressure.
Still another technique, which previously has sometimes been employed, involves sensing the forward velocity of the screw which is operative to cause material to be fed to the mold. The difficulty with this approach however is that a change in the forward velocity of the screw may not necessarily arise from the fact that the mold cavity has become filled with material. Rather this change may be attributable to the existence of other factors affecting the flow of the material intermediate the point at which the material leaves engagement with the screw and the point at which the material enters the mold cavity. More particularly, since cavity pressure is not being determined from readings taken directly within the cavity, but instead is based on readings obtained at a point remote therefrom, other factors such as a blockage in the path of flow of the material, etc. may actually be the cause for a change occurring in the forward velocity of the screw rather than the fact that the cavity has filled with material. Thus, utilizing this technique there exists opportunities for inaccurately portraying the actual condition of the cavity pressure.
Other techniques have been employed heretofore in the prior art, however, these other techniques have also suffered from one or more disadvantages. There has thus existed a need to provide a method, which does not suffer from the disadvantages which characterize prior art methods of controlling cavity pressure, but which would be operable for purposes of determining the profile of a molded part as a means of checking the quality of a given molded part and/or as a basis for effecting adjustments in the molding machine controls. In accord with the present invention, such a method has been provided. The latter method involves the simultaneous taking of a plurality of pressure sensings in a molding machine, and the employment of these pressure sensings in accord with a known mathematical formula to establish a profile of the part being molded wherefrom it is possible to determine the physical characteristics that the molded part will possess. In establishing the profile of the molded part, the values of the pressure sensed in pounds per square inch are plotted against the distance measured from the gate of the molded cavity to the location whereat the pressure sensings were obtained, and with the distance being indicated in units of length.
There are a number of ways in which the method may be practiced. Namely, in accord with one form thereof, the profile of a part being molded in a molding machine having a single mold cavity may be determined by simultaneously taking a pressure sensing in the hot runner of the mold and a pressure sensing at a location in the mold cavity. Also, the profile of a part being molded in a molding machine having a single mold cavity may be determined by obtaining pressure sensings simultaneously from two locations suitably spaced apart in the mold cavity. In addition, the method of the present invention may be employed with molding machines embodying multiple mold cavities to obtain a comparison between the profile of a part being molded in one mold cavity with that being molded in a different mold cavity of the same molding machine based on pressure sensings obtained simultaneously in the corresponding hot runner for each mold cavity. Moreover, the method of the present invention additionally is capable of being practiced to effect a comparison of the profile of parts being molded in multiple cavities of the same molding machine by obtaining pressure sensings simultaneously at differently spaced locations in each of the corresponding mold cavities. Yet another use to which the method of the present invention may be put is for determining whether any material flow has occurred at a given location either in a molding machine embodying a single mold cavity or in a molding machine embodying multiple mold cavities.
Accordingly, it is an object of the present invention to provide a novel and improved method for determining the profile of a molded part as a means of checking the quality of a given molded part.
It is also an object of the present invention to provide such a method for determining the profile of a molded part as a means of establishing whether a need exists to effect adjustments in the molding machine controls.
It is another object of the present invention to provide such a method for determining the profile of a molded part being molded in a molding machine having a single mold cavity by simultaneously taking a pressure sensing in the hot runner of the mold and a pressure sensing at a location in the mold cavity.
A further object of the present invention is to provide such a method for determining the profile of a molded part being molded in a molding machine having a single mold cavity by simultaneously obtaining pressure sensings from at least two locations suitably spaced apart in the mold cavity.
A still further object of the present invention is to provide such a method for determining the profile of molded parts being molded in a molding machine having multiple mold cavities by comparing the profile of a part being molded in one cavity with that being molded in a different mold cavity of the same molding machine based on pressure sensings obtained simultaneously in the corresponding hot runner for each mold cavity.
Yet another object of the present invention is to provide such a method for determining the profile of molded parts being molded in a molding machine having multiple mold cavities by obtaining pressure sensings simultaneously at differently spaced locations in each of the corresponding mold cavities.
Yet still another object of the present invention is to provide such a method for determining the profile of a molded part which is also operable for purposes of determining whether any material flow has occurred at a given location either in a molding machine embodying a single mold cavity or in a molding machine embodying multiple mold cavities.